The present invention relates to a MOS-type variable capacitance element and a voltage control oscillation circuit.
Conventionally, there has been proposed a MOS-type variable capacitance element of a voltage controlled oscillator which is used in a RF field such as communication. FIG. 22 is a cross-sectional view of a conventional MOS-type variable capacitance element. As shown in FIG. 22, the MOS-type variable capacitance element 100 is configured such that an N well 111 is formed on a P-type Si substrate 110, an N+ diffusion region 112 and P+ diffusion regions 113, 114 are formed on a surface of the N well 111 and, further, a gate electrode 115 is formed on the P+ diffusion regions 113, 114 by way of a gate oxide film.
A control voltage terminal 116 is connected to the N+ diffusion region 112 and the P+ diffusion regions 113, 114. The terminal 117 is connected with the oscillation circuit and the gate electrode 115. The P+ diffusion regions 113, 114 form a source electrode and a drain electrode. The N+ diffusion region 112 forms an N-well electrode. In the MOS-type variable capacitance element, by controlling a voltage applied to the control voltage terminal 116, a voltage between the gate electrode 115 and the N well 111 is changed whereby a capacitance value between the gate electrode 115 and the N well 111 is changed.
National Publication of Translated Version of PCT Application 2001-516955 (patent literature 1) proposes a technique similar to the MOS-type variable capacitance element shown FIG. 22.